In Tuesday's Republican U.S. Senate runoff, bombastic conservative Christian candidate Roy Moore faces the appointed, business-backed incumbent Sen. Luther Strange. Despite Strange's support from the U.S. Chamber of Commerce, GOP Senate leaders and President Donald Trump, Moore has consistently led in the polls.

If he wins the nomination and then the December general election, mainstream Republicans warn he would add a rebellious voice to a 52-member GOP Senate caucus. Majority Leader Mitch McConnell already has scant margin for error as he attempts to steer the tax reform debate to a more successful conclusion than the GOP has reached on health care.

There's even concern, improbable as it sounds, that Moore could prove controversial enough to lose in December against Democratic nominee Doug Jones. Moore has twice been forced from judicial office in Alabama for refusing to follow federal court rulings on church-state separation and same-sex marriage.

"It may put the seat in play if Moore wins," said Whit Ayres, a prominent Virginia-based GOP pollster. "I know it sounds ridiculous to think of a Democrat winning Alabama these days, but it also sounded ridiculous to think that Scott Brown could win in Massachusetts."

Ayres referred to the shocking 2010 Massachusetts special election that deprived President Barack Obama and Democrats of a critical Senate seat. That represents the ultimate GOP nightmare in the race to fill out the term of Jeff Sessions, who resigned the seat to become Trump's attorney general.

Even short of that, GOP strategists fear Alabama could complicate preservation of the party's Senate majority in 2018 midterm elections. A Moore victory could embolden less-electable populist challengers to other Republican senators, risking a repeat of unexpected 2010 and 2012 losses in red-leaning states.

Moreover, Moore's flamboyance and hard-right social views would make him a reliable magnet for national attention. The resulting publicity would represent one more obstacle to the GOP's desire to modernize and expand its lagging appeal among women and young people, among other groups.

Despite the president's stated public preference for Strange, the Trump factor in the race has been muddled. As Trump's base among white working-class conservatives remains aloof from the incumbent, ousted White House officials Steve Bannon and Sebastian Gorka have campaigned for Moore in their bid to counter more moderate voices in Congress and the administration.

Housing and Urban Development Secretary Ben Carson has also praised Moore. Remarkably, so has the president, who told a rally in Alabama last week that he may have made a mistake and would help Moore's campaign if Strange loses.

The realclearpolitics.com average of public polls shows the incumbent approaching the Tuesday runoff behind by 10 percentage points. But low-turnout special elections are notoriously unpredictable, and GOP analysts say heavy spending by Strange's establishment allies has left him within striking distance.

The chamber, says top political strategist Scott Reed, is using novel employer-to-employee turnout techniques with Alabama business organizations in hopes to pulling out the victory. With Washington's business agenda on the line, he casts the election as an important precursor to 2018.

A Moore win "would open up the floodgates for others," Reed explained. "Got to shut it down, or else."